GRAs have many applications and, for example, may be employed to drive aircraft flight control surfaces; however, GRAs would be even more extensively used if concerns about potential loss of flight surface control due to gear mesh jams could be alleviated.
In, for example, U.S. Pat. No. 4,856,379, a nonjamming rotary mechanical actuator is proposed which includes at least one moving ring gear having an internal gear surface, with a fixed ring gear on each side of the at least one moving ring gear. Each fixed ring gear is provided with an internal gear surface defining a ring gear bore. A shaft extends through the ring gears, with the shaft including a first cam member rotatable with and secured thereon, and with an axis of the first cam member be offset from an axis of the shaft by a first amount of offset. The ring gears are maintained in a desired relationship by a retaining means which are rotatable with the shaft. Bearing means are mounted within the retaining means for rotatably supporting the shaft, and a generally cylindrical second cam member is provided having a bore extending therethrough. An axis of the bore is offset from an axis of the second cam member by a second amount of offset, with the first cam member being disposed within the bore of the second cam member and having a first amount of offset thereof approximately equal to the second amount of offset. A shear means transmits a torque from the first cam member to the second cam member below a desired torque level. A needle bearing is located around the second cam member, and a compound gear is secured over the needle bearing. The compound gear is provided with a number of gear tooth surfaces thereon, with the number of gear tooth surfaces being equal to a total number of fixed and moving gear rings and in driving relationship therewith below the desired torque level of the shear means.
While the above patented construction offers a solution, the solution has several shortcomings. More particularly, in the patented construction, upon sensing an increased torque in the event of a jam, an abnormal torque is utilized to shear a drive key to free the input shaft to continue to drive the other actuators in the system. An eccentric arrangement is employed to disengage a single planet gear from the output ring gears in the event of a jam to disconnect the actuator's torque path to the control surfaces. The provision of a shear key at the input of the high ratio actuator does not offer a very controllable shear setting and may be fatigue prone. Moreover, the provision of only a single planet provides for a one load path to react output torque, and, therefore, is less redundant, larger, and heavier than conventional multiple load path GRAs. Yet a further disadvantage of the above patented system resides in the fact that the planetary gear arrangement is inherently less efficient than conventional GARs for a given actuator gear ratio.
Examples of other types of torque responsive overload arrangements are proposed in, for example, U.S. Pat. Nos. 4,365,962, 3,499,511, 3,898,817, 3,968,705, 4,282,776, 4,601,218, and, 4,189,960.
Additionally, in commonly assigned U.S. application Ser. No. 07/590,927, entitled Jam Tolerant Rotary Actuator for Multiple Actuator Systems with a Single Prime Mover, filed on even date herewith, an alternative approach or solution for flight control surfaces utilizing multiple actuators is proposed wherein a compound planetary gearing is coupled in a manner so as to provide for speed compensation in the event of a gear mesh jam without any loss of position authority or torque capacity.